Microparticles have been used as a solid phase in flow cytometric assays (e.g., U.S. Pat. No. 5,981,180, hereby incorporated by reference). These methods employ levels of fluorescence in the particles to identify different capture reagents. Fluorescence-coded particles have been attached to the ends of optical fibers for multiplex analyses (e.g. U.S. Pat. No. 6,023,540, hereby incorporated by reference). Microparticles have also been embedded in glass fiber filters (e.g. U.S. Pat. Nos. 5,356,785 and 5,879,881, hereby incorporated by reference), sedimented on the bottoms of microtiter wells, packed into microchannels, and attached to planar glass substrates (e.g., U.S. Pat. No. 6,133,436, hereby incorporated by reference). Single-particle assays have also been developed.
These methods are susceptible to a number of problems. When capture molecules are deposited in arrays on solid planar substrates, the volume of reagent and the efficiency of coupling can vary widely from spot to spot. The specificity and affinity of binding to the target can also be reduced from solution-phase values. When the target solution is contacted with a planar substrate, capture can be slow because a diffusion boundary layer develops. The intensity of the captured target can be reduced both because of transport-limited kinetics and the relatively large capture area.
What is needed are methods and compositions for detecting targets with increased sensitivity, and the ability to perform multiple assays simultaneously.